Structural and electronic properties of BiOX (X = F, Cl, Br, I) considering Bi 5f states

Density functional theory (DFT) calculations have been performed to investigate the structural and electronic properties of BiOX (X = F, Cl, Br, I) photocatalysts, considering the Bi 5f states. Comparing with our previous results without Bi 5f states, the present work conveys similar gap features and comparable gap widths for the unrelaxed species, but the relaxation here causes opposite directions of atomic displacements and different trends for band and gap widths. The unrelaxed BiOF exhibits a direct band gap of 3.25 eV, while the other three species present the indirect gaps of 2.62, 2.16 and 1.51, respectively. Atomic relaxation expands the above gaps to 3.41, 2.69, 2.21 and 1.62 eV correspondingly. Analyses based on wavefunction isosurfaces and decomposed orbitals reveal that at the valence-band tops, O 2p_z and F 2p_z states prevail in BiOF, O 2p_y and X np_y dominate in BiOCl and BiOBr, and the I 5p_x state governs in BiOI. The conduction-band bottoms are composed mainly of the Bi 6p_z orbital.     

Electronic structure and transport properties of Co(Si1−xMx) (M = Al, P): First-principles study

In this study, by using the full-potential linear augmented plane wave (FLAPW) method based on the density functional theory (DFT), the lattice parameter of CoSi was calculated theoretically and the calculations of the electronic structures of CoSi and CoSi_1−xM_x (M = Al, P and x = 0.03125, 0.125) were performed. The calculated lattice parameter of binary CoSi is about 0.27% smaller than the experimental value. Calculated electronic structures show that CoSi is a semi-metal and the density of states (DOS) is very small at the Fermi level. M-doping can tune the Fermi level and the hole pockets and the electron ones, which is very valuable to modulate the transport properties. Based on the calculated electronic structures and our experimental results on CoSi [C.C. Li, W.L. Ren, L.T. Zhang, K. Ito, J.S. Wu, J. Appl. Phys. 98 (2005) 063706], the intrinsic relations between electronic structures and transport properties of CoSi and CoSi_1−xAl_x are discussed in detail. The transport properties along main crystallographic directions of binary CoSi and CoSi_1−xAl_x are experimentally examined. The experimental results show that the electrical resistivity of CoSi-based compounds is anisotropic, while the Seebeck coefficient is almost isotropic. The calculated band structures of CoSi_1−xAl_x can theoretically interpret the anisotropy of the electrical transport properties.     

基于BiOX(X=Cl、Br、I)新型高性能光催化材料的最新研究进展

铋系半导体BiOX(X=Cl、Br、I)因其独特的层状结构和合适的禁带宽度而表现出优异的光催化活性与稳定性, 已成为光催化材料领域极具应用前景的材料体系。本文首先针对BiOX(X=Cl、Br、I)光催化材料研究中的关键科学问题进行了深入分析, 进一步综述了国内外解决上述关键问题所采取的有效措施, 包括: 微结构调控、半导体复合、贵金属沉积、离子掺杂和表面敏化等, 并针对纳米结构BiOX(X=Cl、Br、I)负载于合适载体上实现固载的研究进展进行了概述, 从而对基于BiOX(X=Cl、Br、I)新型高性能光催化材料的最新研究进展进行了全面深入的综述, 最后展望了BiOX光催化材料的研究方向与趋势。     

First principles calculations of the electronic, dynamical, and thermodynamic properties of the rocksalt ScX (X = N, P, As, Sb)

We have investigated the electronic, dynamical, and thermodynamic properties of the rocksalt ScX (X = N, P, As, Sb) using a plane-wave pseudopotential method within the generalized gradient approximation in the frame of density functional perturbation theory. The calculated lattice constants are found to differ by less than 0.56% from the available experimental values. These materials have the indirect Γ–X band gaps and a wide and direct band gap at the X-point in band structure, which are closer to experimental results than the previous calculations. A linear-response approach is used to calculate the phonon frequencies, the phonon density of states and LO–TO splitting. The obtained phonon frequencies at the zone-center (Γ-point) for the Raman-active and infrared-active modes are analyzed. We also calculate the thermodynamic functions using the phonon density of states, and the calculated values are in nearly perfect agreement with experimental data.     

Remarkable Piezophoto Coupling Catalysis Behavior of BiOX/BaTiO3 (X = Cl,Br, Cl0.166Br0.834) Piezoelectric Composites

Polarization field engineering of piezoelectric materials is considered as an advisable strategy in fine‐tuning photocatalytic performance which has drawn much attention recently. However, the efficient charge separation that determines the photocatalytic reactivities of these materials is quite restricted. Herein, a judicious combination of piezoelectric and photocatalytic performances of BiOX/BaTiO₃ (X = Cl, Br, Cl_0.166Br_0.834) to enable a high piezophotocatalytic activity is demonstrated. Under the synergic advantages of chemical potential difference and piezoelectric potential difference in BiOX/BaTiO₃ composites, the photoinduced carriers recombination is largely halted, which directly contributes to the significantly promoted piezophotocatalytic activity of piezoelectric composites. Inspiringly, the BiOBr/BaTiO₃ composites under light irradiation with auxiliary ultrasonic activation result in an ultrahigh and stable photocatalytic performance, which is much higher than the total of those by isolated photocatalysis and piezocatalysis, and can rival current excellent photocatalytic system. In fact, the theoretical piezoelectric potential difference of BiOBr/BaTiO₃ composites reaches 100 mV, which far exceeds the pure BaTiO₃ of 31.21 mV and BiOBr of 30 mV, respectively. First, fabrication of BiOX/BaTiO₃ piezoelectric composites and its remarkable piezophoto coupling catalysis behavior lays new ground for developing high‐efficiency piezoelectric photocatalysts in purifying wastewater, killing bacteria, and other piezophototronic processes.     

Structural and electronic properties of AlX (X = P,As, Sb) nanowires: Ab initio study

Present paper discusses the structural stability and electronic properties of AlX (X = P, As and Sb) nanowires in its linear, zigzag, ladder, square and hexagonal type atomic configurations. The structural optimization has been performed in self consistence manner by using generalized gradient approximation with revised Perdew, Burke and Ernzerhof type parameterization. The study observes that in all the three nanowires, the square shaped atomic configuration is the most stable one. The calculated electronic band structures and density of states profile confirms the semiconducting behaviour of linear and zigzag shaped nanowires of AlP, whereas for AlAs and AlSb nanowires are metallic. The ground state properties have also been analysed in terms of bond length, bulk modulus and pressure derivative for all the nanowires along with their bulk counterpart. The lower bulk modulus of all the linear shaped geometries of AlX nanowires in comparison to its bulk counterpart indicates softening of the material at reduced dimension.     

Design of XS2 (X = W or Mo)-Decorated VS2 Hybrid Nano-Architectures with Abundant Active Edge Sites for High-Rate Asymmetric Supercapacitors and Hydrogen Evolution Reactions

Two-dimensional layered transition metal dichalcogenides have emerged as promising materials for supercapacitors and hydrogen evolution reaction (HER) applications. Herein, the molybdenum sulfide (MoS₂)@vanadium sulfide (VS₂) and tungsten sulfide (WS₂)@VS₂ hybrid nano-architectures prepared via a facile one-step hydrothermal approach is reported. Hierarchical hybrids lead to rich exposed active edge sites, tuned porous nanopetals-decorated morphologies, and high intrinsic activity owing to the strong interfacial interaction between the two materials. Fabricated supercapacitors using MoS₂@VS₂ and WS₂@VS₂ electrodes exhibit high specific capacitances of 513 and 615 F g⁻¹, respectively, at an applied current of 2.5 A g⁻¹ by the three-electrode configuration. The asymmetric device fabricated using WS₂@VS₂ electrode exhibits a high specific capacitance of 222 F g⁻¹ at an applied current of 2.5 A g⁻¹ with the specific energy of 52 Wh kg⁻¹ at a specific power of 1 kW kg⁻¹. For HER, the WS₂@VS₂ catalyst shows noble characteristics with an overpotential of 56 mV to yield 10 mA cm⁻², a Tafel slope of 39 mV dec⁻¹, and an exchange current density of 1.73 mA cm⁻². In addition, density functional theory calculations are used to evaluate the durable heterostructure formation and adsorption of hydrogen atom on the various accessible sites of MoS₂@VS₂ and WS₂@VS₂ heterostructures.     

Luminescence excitation spectra of TbX3 (X = Cl, Br, I)

A systematic study of the excitation spectrum of TbX₃ (X = Cl⁻, Br⁻, I⁻) is presented in this work. In general, the excitation spectra of TbX₃ can be divided into three major regions: (1) the short-wave host lattice absorption region, (2) the intermediate absorption region where the Tb³⁺ 4f⁸ → 4f⁷5d¹ interconfigurational excitation transition are located, and (3) the long-wave excitation region where the Tb³⁺ 4f⁸ → 4f⁸ intraconfigurational excitation transition are located. The high spin and the low spin components of the Tb³⁺ interconfigurational excitation transition are clearly identified in the case of TbCl₃. The luminescence of TbX₃ (X = Cl⁻, Br⁻, I⁻) is dominated by emission transitions emanating from the Tb³⁺⁵D₄ state. A comparative study of the optical properties of TbX₃ (X = Cl⁻, Br⁻, I⁻) with the properties of the Tb³⁺ ion in several halide host lattices is presented. Further, a comparative study of the fundamental host lattice optical transitions in terbium halides and other halide materials is also presented.     

Ab initio study of the binding of collagen amino acids to graphene and A-doped (A = H, Ca) graphene

We present a theoretical study of the binding of collagen amino acids (AA, namely glycine, Gly; proline, Pro; and hydroxyproline, Hyp) to graphene (Gr), Ca-doped graphene and graphane (Gra) using density functional theory calculations and ab initio molecular dynamics (AIMD) simulations. It is found that binding of Gly, Pro and Hyp to Gr and Gra is thermodynamically favorable yet dependent on the amino acid orientation and always very weak (adsorption energies E_ads range from −90 to −20 meV). AIMD simulations reveal that room-temperature thermal excitations are enough to induce detachment of Gly and Pro from Gr and of all three amino acids from Gra. Interestingly, we show that collagen AA binding to Gr is enhanced dramatically by doping the carbon surface with calcium atoms (corresponding E_ads values decrease by practically two orders of magnitude with respect to the non-doped case). This effect is result of electronic charge transfers from the Ca impurity (donor) to Gr (acceptor) and the carboxyl group (COOH) of the amino acid (acceptor). The possibility of using Gr and Gra as nanoframes for sensing of collagen amino acids has also been investigated by performing electronic density of states analysis. It is found that, whether Gr is hardly sensitive, the electronic band gap of Gra can be modulated by attaching different number and species of AAs onto it. The results presented in this work provide fundamental insights on the quantum interactions of collagen protein components with carbon-based nanostructures and can be useful for developments in bio and nanotechnology fields.     

Ultrafast third-order optical non-linearity of 0.56GeS2–0.24Ga2S3–0.2KX(X = Cl, Br, I) chalcohalide glasses by femtosecond Optical Kerr Effect

0.56GeS₂–0.24Ga₂S₃–0.2KX(X = Cl, Br, I) chalcohalide glasses were prepared and their third-order optical non-linearities χ⁽³⁾ have been studied systematically using the femtosecond time-resolved Optical Kerr Effect technique at wavelength of 800 nm. In this system, 0.56GeS₂–0.24Ga₂S₃–0.2KCl glass shows the largest χ⁽³⁾ (1.82 × 10⁻¹³ esu), but 0.56GeS₂–0.24Ga₂S₃–0.2KBr glass has the fastest optical non-linear response time in subpicosecond domain (about 340 fs), which is due to the ultrafast distortion of the electron clouds surrounding the balanced position of Ge, Ga, S, K and Br atoms. The local hyperpolarizability determining non-linear optical response are determined by the partially ionic bonds originating from Ge(Ga)–S bonds and halogen valence bonds. They show great potential applications on the glass-based optoelectronic devices like optical switching.     

Density of electronic states and dispersion of optical functions of defect chalcopyrite CdGa2X4 (X = S,Se): DFT study

A density functional theory (DFT) based on full potential linear augmented plane wave (FPLAPW) was used for calculating the electronic structure, charge density and optical properties of CdGa₂X₄ (X = S, Se) compounds. Local density approximation (LDA), generalized gradient approximation (GGA), Engle Vasko generalized gradient approximation (EVGGA) and recently modified Becke–Johnson (mBJ) were applied to calculate the band structure, total and partial density of states. The investigation of band structures and density of states of CdGa₂X₄ (X = S, Se) elucidate that mBJ potential show close agreement to the experimental results. The mBJ potential was selected for further explanation of optical properties of CdGa₂X₄ (X = S, Se). The study of electronic charge density contours shows that change in the bond lengths and bond nature affect the band gap of the compounds. The two non-zero dielectric tensor components and its derivatives show considerable anisotropy between the perpendicular and parallel components. The present work provide accurate information about the combination (hybridization) of orbital, formation of bands and dispersion of non-zero tensor components of CdGa₂X₄ (X = S, Se).     

Structure and electronic properties of transition metal dichalcogenide MX2 (M = Mo,W, Nb; X = S,Se) monolayers with grain boundaries

Layered transition metal dichalcogenides with unique mechanical, electronic, optical, and chemical properties can be used for novel nanoelectronic and optoelectronic devices. Large-area monolayers synthesized using chemical vapor deposition are often polycrystals with many dislocations and grain boundaries (GBs). In the present paper, atomic structure and electronic properties of MX₂ (M = Mo, W, Nb; X = S, Se) with the GBs were investigated using first principles based on density functional theory. Simulation results revealed that the zigzag-oriented GBs (which consist of pentagon/heptagons (5-7) pairs) were more stable than the armchair-oriented GBs (which consist of pentagon/heptagons (5-7-5-7) pairs). The GBs induced defect levels are located within the band gap for the semiconductor materials of MX₂ (M = Mo, W; X = S, Se) monolayers, and the NbS₂ and NbSe₂ remained as metallic materials with GBs. Results provided a possible pathway to build these nano-layered materials into nanoelectronic devices.     

Scintillation properties of CsSrX3:Eu (CsSr1−yEuyX3, X = Cl,Br; 0 ≤ y ≤ 0.05) single crystals grown by the Bridgman method

The single crystals of CsSr_1−yEu_yCl₃ and CsSr_1−yEu_yBr₃ (0 ≤ y ≤ 0.05) grown by the Bridgman method have been investigated for their scintillation properties. The radioluminescence spectra of the crystals demonstrated a narrow band at 400–500 nm which peak position shifts continuously toward larger wavelength as the Eu²⁺ concentration increases. The scintillation light yield achieves maximal value of 33,400 ± 1700 photons per MeV with energy resolution of 11.5% at CsSr_0.95Eu_0.05Cl₃ and 31,300 ± 1600 photons per MeV with energy resolution of 9% at CsSr_0.95Eu_0.05Br₃. The decay times at room temperature are 2.7 ± 0.2 μs and 2.5 ± 0.2 μs, respectively.     

PbX2（X：I,Br,Cl）基非氟卤化物玻璃的形成行为

为了获得具有实用意义的非氟卤化物玻璃，有必要探索新的玻璃形成系统，本文获得了 新的Ｃ ＰｂＸ２（Ｘ：Ｉ，Ｂｒ，Ｃｌ）为基质的三元非氟卤化物玻璃形成系统，并且首次发现ＰｂＩ２可以单独形成玻璃，所得的三元玻璃形成系统均表现出了良好的玻璃形成能力，此外，还从晶体结构，化学键的特怀，离了半径应电负性等角度探讨了各卤化物在玻璃形成过程中所坊的作用，结果表明，这些因素对玻璃形成均具有重要的作用。     

Photoinduced non-linear optical monitoring of novel semimagnetic semiconductors Pb1−xYbxX (X = S,Se, Te)

It was revealed that photoinduced non-linear optical methods may be used as a sensitive tool for investigations of low temperature semiconductor-isolator phase transformation. This was experimentally demonstrated using a method of photoinduced optical second harmonic generation, where the photoinducing regime was done by polarized nitrogen laser. The physical insight of the method is explained by the interaction of polarized optical field with boson sub-systems (phonon and magnons) near temperature of the phase transformation. Comparing low temperature dependence of specific heat for Pb_1−xYbX (X = S, Se, Te) with the ones for the photoinduced non-linear optical methods we have found substantially better temperature recognition of corresponding phase transformations and the output non-linear optical signal is inversely proportional to the heat capacity renormalized by temperature — C_p / T. The obtained results demonstrate high sensitivity of the photoinduced SHG to the low temperature phase transformations.     

First principles study on the structural properties and electronic structure of X2B (X = Cr, Mn, Fe, Co, Ni, Mo and W) compounds

First principles calculations are performed to study the stability, electronic and structural properties of X₂B (X = Cr, Mn, Fe, Co, Ni, Mo and W). The calculated cohesive energy and formation enthalpy of these compounds both have negative values, which indicate that they are thermodynamically stable structures. The ground states of Cr₂B and Mn₂B are anti-ferromagnetic; Fe₂B and Co₂B are ferromagnetic; Ni₂B, Mo₂B and W₂B are paramagnetic. The calculated local magnetic of Fe₂B is 1.962μ_B/Fe, and for Co₂B is 1.182μ_B/Co. They are comparable to the values of Fe₃B (1.97μ_B/Fe) and Co₃B (1.18μ_B/Co), but smaller than pure Fe and Co. The observed magnetic behaviors of X₂B compounds can be explained by Stoner’s model. Two main peaks are observed in the calculated PDOS (partial density of states) of these compounds (P1 and P2). P1 is caused by strong covalent X–B bonds and P2 is attributed to metallic X–X bonds.     

Ab initio study of stacking faults and deformation mechanism in C15 Laves phases Cr2X (X = Nb,Zr, Hf)

Based on the synchroshear model, the formation of stacking fault and twinning fault in C15 Laves phases is modeled, then the generalized stacking fault energy curves and deformation mechanism in C15 Laves phases Cr₂X (X = Nb, Zr, Hf) alloys are investigated by ab initio calculations based on the density functional theory. The results demonstrate that the unstable stacking fault and twinning fault energies of C15 Laves phases Cr₂X (X = Nb, Zr, Hf) by the synchroshear are still large while the stable stacking fault and twinning fault energies are low, and the deformation modes by extended partial dislocation and twining are feasible in C15 Laves phases Cr₂X (X = Nb, Zr, Hf). Moreover, the Cr₂Nb has the largest deformation twinning tendency, followed by Cr₂Zr and Cr₂Hf. The evolution of electronic structure during the synchroshear process is further studied to unveil the intrinsic mechanism for the formation of stacking fault and twinning fault in C15 Laves phases Cr₂X (X = Nb, Zr, Hf).     

Cu magic angle spinning (MAS) nuclear magnetic resonance (NMR) study of CuX crystals (X=Cl, Br, and I) and CuX-based glasses (X=Cl, Br, and I)

⁶³Cu MAS NMR spectra of CuX crystals (X=Cl, Br, and I) and CuX-based glasses (X=Cl, Br, and I) have been measured. The CuCl and CuI crystals gave the isotropic chemical shift values around 0 ppm, and the CuBr crystal, around −55 ppm. The peak positions of the chlorocuprate(I), bromocuprate(I), and iodocuprate(I) glasses were very close to those of the CuX crystals, respectively. This result indicates that these glasses mainly consist of CuX₄ tetrahedra (X=Cl, Br, and I). The halogen coordination environments around Cu⁺ in bromochlorocuprate(I) glasses were dependent on the kind of modifying cation and the Br⁻/(Cl⁻+Br⁻) ratio. The bromochlorocuprate(I) glasses were mainly composed of CuCl_mBr_4−m or CuCl_nBr_4−n tetrahedra (m=0, 1, 2, and 4; n=0, 1, and 4).